1. Field of the Invention
This invention relates to a sheet conveying unit used to accurately convey a sheet such as photosensitive film for example, in an image recording apparatus, an image reading apparatus or the like.
2. Description of the Prior Art
FIG. 5 of the accompanying drawings shows an example of the sub-scanning conveying mechanism of an apparatus described in Japanese Laid-Open Patent Application No. 63-212645 which reads an accumulative fluorescent material sheet on which radiation images are accumulatively recorded.
In FIG. 5, the reference numeral 101 designates an accumulative fluorescent material sheet which is conveyed rightwardly as viewed in the figure, whereby sub-scanning is effected and at the same time, a light beam 113 scans perpendicularly to the plane of the drawing sheet, whereby main scanning is effected and the reading of images is done by exciting light. The sheet 101 is nipped between two pairs of sub-scanning rollers 102, 103 and 104, 105. The reference numeral 108 denotes a motor, the power of which is transmitted to the sub-scanning roller 102 by a belt passed over a pulley 109 coupled to the output shaft of the motor and a pulley, not shown, mounted on the shaft of the sub-scanning roller 102. Pulleys 106 and 107 are also coupled to the shafts of the sub-scanning rollers 102 and 104 and are connected together by a belt 114. The sheet between the two pairs of sub-scanning rollers is supported by guide plates 111 and 112. With the construction as described above, the motor is rotated clockwise, whereby the clockwise rotational force of the motor is transmitted to the sub-scanning rollers 102 and 104 and the sheet 101 can be conveyed rightwardly as viewed in the figure.
FIG. 6 of the accompanying drawings shows a sub-scanning conveying mechanism resembling the above-described example of the prior art which is described in Japanese Laid-Open Patent Application No. 63-258339. FIG. 6 corresponds to a view in which the example of the prior art shown in FIG. 5 is seen from above, and in FIG. 6, reference numerals identical to those in FIG. 5 designate identical or equivalent members.
The difference of the example shown in FIG. 6 from the example shown in FIG. 5 is that the driving power of the motor 108 is transmitted not by the use of the belt 110, but by the use of a connecting member 120. In FIG. 6, the reference numeral 121 designates bearings, and the reference numerals 122 and 123 denote frames.
However, these examples of the prior art have suffered from the problem that the constant speed of the rotational speed of the sub-scanning rollers is deteriorated relative to the constant rotational speed of the drive source.
In optical image reading or recording apparatuses of high accuracy like the above-described examples of the prior art, a constant speed of the conveyance speed in the sub-scanning direction of a sheet is very important. Particularly in a recording apparatus for medical treatment, the pitch irregularity of scanning lines caused by the disturbance of the conveyance speed leads to the density irregularity of images, and in the worst case, may lead to a wrong diagnosis. Consequently, it is necessary that the wow flutter of the conveyance speed of the sheet be held down to the order of 2% or less.
If the conveyance speed of the sheet is 40-50 mm/sec. and the diameter of the sub-scanning roller 102 is 20-40 mm and the reduction ratio of the sub-scanning rollers and the motor 108 is 1:1, the number of revolutions of the motor is 20-50 rpm. It is very difficult to hold down the wow flutter to 0.2% or less as previously described in such a low rotation range, and there is required a motor controlling technique of very high accuracy.
Also, the sheet conveyed is often of a large size such as size B4 or a half size (43 cm.times.35 cm), for example, in the case of the use for medical treatment, and this makes it even more difficult to obtain a constant conveyance speed.
It is reported in the aforementioned Japanese Laid-Open Patent Application No. 63-212645 that where power is transmitted by the use of a belt as in FIG. 5, the power spectrum of the driving shaft and the follower shaft is such as shown in FIG. 7 of the accompanying drawings. That is, it is seen that even in the best case, the follower shaft has less speed consistency than the driving shaft.
Also, where a connecting member is used as shown in FIG. 6, the speed consistency of the rollers may be less than that of the motor. For example, where a popular metallic flexible coupling is used as the connecting member, the resonance frequency of the sub-scanning rollers is determined by the inertial mass of the sub-scanning rollers and the torsion spring constant of the flexible coupling. The wow flutter of the rollers in the vicinity of this resonance frequency is remarkably aggravated. Moreover, it has been found that unless a great inertial mass such as a fly-wheel is added to the sub-scanning rollers, there is a possibility of said resonance frequency being in an area which may cause harmful scanning line pitch irregularity.
As noted above, the above-described examples of the prior art suffer from the possibility of conveying the sheet at an inconsistent speed.
Also, speed consistency can be measured only after the apparatus has been assembled and therefore, quality control is difficult. Further, the interchange of the rollers and motor is not easy and it is difficult to say that the speed consistency is reproduced after the interchange.